Arizona Department of Water Resources Hydrology Division Application of the Prescott Active Management Area Groundwater Flow Model Planning Scenario 1999-2025 Modeling Report No. 12 September, 2002 By Keith Nelson EXECUTIVE SUMMARY In 1995, the Arizona Department of Water Resources developed a regional groundwater flow model to quantify the impacts of groundwater pumpage and recharge in the Prescott Active Management Area (Corkhill and Mason, 1995). The model has been updated with new hydrogeologic data and revised estimates of historical water-use and recharge. The updated model was calibrated to measured groundwater level and natural groundwater discharge targets, and was evaluated by a sensitivity analysis for steady state and transient simulations (1939- 1999). In addition, the model was used to simulate projected hydrologic conditions, including groundwater levels and natural groundwater discharge, between 1999 and 2025. The ADWR’s Prescott Active Management Area includes 485 square miles in central Yavapia County and includes the Little Chino and Upper Agua Fria sub-basins which discharge groundwater to the Verde and Agua Fria Rivers, respectively. The model area covers about 220 square miles of the Upper Agua Fria and Little Chino sub-basins and includes the areas where most of the groundwater pumpage and recharge occur. Historical groundwater pumpage has reduced groundwater levels in most parts of the Upper Alluvial Unit and Lower Volcanic Unit aquifers, and has modified natural groundwater discharge out of the sub-basins. Model simulations have approximately replicated observed groundwater levels and natural groundwater discharge through the transient simulation (1939 to 1999) and through the beginning of the planning scenario (1999 to 2002). One planning scenario was simulated from 1999 to 2053 to assess the hydrologic impacts of projected groundwater withdrawals and recharge. However, projection results were only assessed through the year 2025, because of an increasing number of dry model cells encountered halfway through the planning simulation. In the planning scenario, groundwater pumpage for municipal, industrial and domestic demands was projected to increase from about 14,000 acre- feet/year in 1999 to about 24,500 acre-feet/year by 2025; agriculture demand was projected to decrease from about 4,000 acre-feet/year in 1999 to about 2,100 acre-feet/year by 2025. Model projection results through 2025 show that continued groundwater pumpage will further exacerbate groundwater level declines in most parts of the model area and that the groundwater discharge rate near Del Rio Springs (spring flow at the surface and subsurface flow) will continue to decrease over time. However, groundwater levels are projected to rise in the southern portion of the Upper Agua Fria sub-basin and result in a gradual increase in groundwater ii discharge at the Agua Fria River if the projected effluent recharge and long-term natural recharge rates hold true. Model results demonstrate that except for years of significant precipitation and associated flood recharge, the Prescott AMA on a regional scale will continue to experience a net loss of groundwater storage. iii ACKNOWLEDGEMENTS I would like to acknowledge those individuals and organizations that supplied assistance or data towards the development of this report. I wish to express my sincere appreciation to Frank Corkhill and Dale Mason for developing the Prescott AMA Groundwater Flow Model and for providing valuable comments for the updated version. In addition, I want to thank all individuals and organizations responsible for the collection of hydrologic data within the model area including field personnel from the Arizona Department of Water Resources, U.S. Geologic Survey and the University of Arizona. I would also like to acknowledge the following individuals who provided institutional and/or technical information about the general model area including Phil Foster, Bill Remick, William Musielak, Jim Holt, Bill Allen, Brad Huza and Larry Tarkowski. I also want to thank Roberto Chavez for creating the figures in this report. iv Table of Contents Executive Summary............................................................................................................ ii Acknowledgements............................................................................................................ iv Chapter 1: Introduction and Background.............................................................................1 1.1. Introduction .....................................................................................................1 1.2. Objective, Scope, Goals...................................................................................1 1.3. Prescott Model Area ........................................................................................2 1.4. Factors Which May Effect Model Results.......................................................2 Chapter 2: Modifications to the Prescott AMA Model........................................................5 Chapter 3: Steady State Simulation of the Prescott AMA Model circa 1939......................7 3.1.Steady State Simulation: Groundwater Pumpage..............................................7 3.2. Steady State Simulation: Recharge...................................................................7 Chapter 4: Transient Simulation of the Prescott AMA Model 1939-1999..........................9 4.1.Transient Simulation: Groundwater Pumpage...................................................9 4.2. Transient Simulation: Recharge......................................................................10 Chapter 5: Results of the Steady State Simulation ............................................................12 Chapter 6: Results of the Transient Simulation .................................................................14 Chapter 7: Results of the Sensitivity Analysis...................................................................17 Chapter 8: Groundwater Conditions in 1999.....................................................................19 Chapter 9: Planning Scenario 1 (PS1) 1999-2053 .............................................................20 9.1. PS1: Groundwater Pumpage...........................................................................20 9.2. PS1: Groundwater Recharge...........................................................................22 Chapter 10:Results of PS1 Simulation...............................................................................24 Chapter 11: Conclusions....................................................................................................26 References..........................................................................................................................27 APPENDIX I - Figures ................................................................................................. 2-12 APPENDIX II – Hydrographs ......................................................................................... 1-9 APPENDIX III – Pictures (Figures) ............................................................................ 13-16 1.1 List of Tables Table No. 1. Flood Recharge Applied to the Prescott AMA Groundwater Model ..........................11 2. Effluent Recharge Applied to the Prescott AMA Groundwater Model.......................11 3. Simulated and Conceptual Steady State Water Budgets..............................................12 4. Statistical Summary of Steady State Error Analysis for the LVU (Layer 2)...............12 5. Simulated and Conceptual Transient Water Budgets (1940-1999) .............................14 6. Combined Statistical Summary of Transient Error Analysis for the UAU (Layer 1) and the LVU (Layer 2).................................................................15 6a Statistical Summary of Transient Error Analysis for the UAU (Layer 1) ...................15 6b Statistical Summary of Transient Error Analysis for the LVU (Layer 2)....................15 7. Sensitivity Analysis Results ........................................................................................18 8. Projected Groundwater Pumping Demands for Planning Scenario 1 (PS1) ...............23 9. Projected Simulated Water Budget for PS1 ................................................................25 1.2 Appendix I and III - List of Figures Figure No. 1. Prescott AMA Model Area ............................................................................................4 2. Areal Distribution of Hydraulic Conductivity in the UAU .........................................30 v 3. Areal Distribution of Hydraulic Conductivity in the LVU..........................................31 4. Simulated heads in the UAU (1999)............................................................................32 5. Simulated depth-to-water in the UAU (1999) .............................................................33 6. Simulated heads in the LVU (1999) ............................................................................34 7. Simulated depth-to-water in the LVU (1999)..............................................................35 8. Difference between Measured and Simulated Water Level (1999).............................36 9. PS1: Simulated heads in the UAU (2025) ...................................................................37 10. PS1: Simulated depth-to-water in the UAU (2025).....................................................38 11. PS1: Simulated heads in the LVU (2025)....................................................................39